A number of organotin maleates forming good heat-stabilization and photostabilization agents for vinyl chloride polymers are known.
Thus, compositions containing organotin maleates, such a dibutylin bis(isooctyl maleate) or dibutylin bis(isooctyl maleate) oxide, are described in the literature. The use of these compositions as heat-stabilizers for halogenated organic compounds, in particular halogenated polymers such as poly(vinyl chloride) (PVC) and chlorinated poly(vinyl chloride) (CPVC), is also well known.
The effectiveness of stabilizers based on organotin maleate for PVC, in particular articles intended for external use, is well known.
Thus, in the article entitled "Worldwide Weathering of Polyvinyl Chloride", by Emery Szabo and Robert Lally; Polymer and Engineering Science, April 1975, Vol. 15, No. 4, the results of studies on long-term exposure to weather are presented. The stabilizing compositions used were compositions containing barium/cadmium soaps, compositions containing organotin maleates and compositions containing organotin mercaptoacetates. It is noted, in the conclusions, that DBTM (dibutyltin maleate) gives the best results as UV stabilizer and that organotin mercaptoacetates give the worst results.
Although the superiority of compositions containing organotin maleates in the stabilization of PVC formulations subjected to weathering has been recognized for a long time, these compositions have not succeeded in fully satisfying market requirements because of many disadvantages.
Thus, British Patent 787,930 explains, from page 1, line 65 to page 2, line 3, that dibutyltin maleate is difficult to disperse and in addition generates, during the conversion of PVC, a volatile maleic anhydride fraction which has lacrimatory and irritating effects on the people handling it. In order to overcome these faults, GB 787,930 provides compositions containing organotin maleate hemiesters which are liquid, such as dibutyltin bis(monobutyl maleate). The presence of the alcohol group of the ester has the effect of decreasing the tin content, causing reduced heat stabilization, but does not remove the volatile fraction generated by the stabilization mechanism, that is to say the formation of the hemiester of maleic acid by the reaction of organotin bis(monoalkyl maleate) with the gaseous HCl generated during the processing of PVC.
U.S. Pat. No. 3,296,289 describes stabilizing compositions containing diorganotin maleate hemiester which are improved insofar as these compositions, composed of diorganotin bis(maleate hemiesters), are solid at room temperature. A typical composition from U.S. Pat. No. 3,296,289 was dibutyltin bis(cyclohexyl maleate), having a melting point of 71.degree. C. to 73.degree. C., as described in Example 1. Although the solid compositions containing organotin maleate hemiester described in U.S. Pat. No. 3,296,289 make it possible to solve some handling problems, they do not for all that remove the irritating volatile compounds generated when the stabilizing compositions are subjected to the heat conditions applied during the processing of PVC, which is carried out at approximately 200.degree. C.
U.S. Pat. No. 3,555,060 relates to organic tin compounds, such as, in particular, dibutyltin bis(isooctyl maleate).
These compounds are complex chelates exhibiting an .tbd.Sn--O--Sn.tbd. bond with 2 tin atoms per molecule.
However, these products are viscous, coloured liquids which are difficult to use.
Another problem which is raised by the use of organotin maleates as heat-stabilizer for vinyl chloride polymers is the difficulty of processing these formulations. Indeed, it is well known, in particular during the extrusion of rigid PVC, that the molten polymer containing the said stabilizers exhibits a strong tendency to adhere to the conversion equipment.
To overcome this problem, use is made of sometimes complex combinations of lubricants and attempts are more rarely made to modify the structure of the stabilizer itself.
Thus, Patent GB 1,378,851 provides for the addition of a mixture of paraffin wax and of a lubricating acrylic polymer to a PVC formulation containing dibutyltin di(methyl maleate), so as to reduce the adhesion to the equipment. The improvement thus obtained is characterized by a greater ease in detaching a film converted on a roll mill but, however, remains unsatisfactory.
However, the significant, indeed excessive, addition of lubricants to PVC formulations can have disadvantages. Thus, an excessively high content of internal lubricant will have, inter alia, consequences of increasing the plasticization of the PVC and thus of decreasing its Vicat point. In the case of rigid formulations, this can be a factor which prohibits the use of organotin maleates.
In the same way, an excess of external lubricant, intended to limit problems of adhesion during extrusion, will have a tendency to promote phenomena of exudation and thus of deposits on the equipment.
Thus, despite more than thirty years of research, heat-stabilizers for PVC based on organotin maleates continue to have a limited use although they exhibit excellent stabilizing properties and very good resistance to weathering. Their properties cannot be regarded as entirely satisfactory:
either because, during processing, they decompose to give volatile products which are irritating and lacrimatory, PA1 or because they are difficult to process. PA1 RA represents PA1 R.sup.1 A represents PA1 an absorption band in the region of 1730 cm.sup.-1 characteristic of ester functional groups, PA1 an absorption band in the region of 1580 cm.sup.-1 characteristic of tin carboxylates, PA1 an absorption band in the region of 680 cm.sup.-1 characteristic of .tbd.Sn--O--Sn.tbd. bonds.